1. Field of the Invention
The present invention relates to a positioning detector for detecting whether a workpiece has been positioned on a reference surface and indicating the detected condition on a indicating unit to allow the user to visually confirm the detected condition.
2. Description of the Related Art
For positioning a workpiece on a reference surface in a machine tool such as a cutting machine, it has been a customary practice to jet a fluid under pressure from a nozzle toward a surface of the workpiece, detect a back pressure developed behind the nozzle by the application of the fluid to the workpiece surface, and determine distance by which the workpiece is spaced from the reference surface based on the generated back pressure.
The basic principles of the detection of the back pressure are illustrated in FIG. 1 of the accompanying drawings. A pressure fluid supplied from an unillustrated pressure source whose pressure is regulated by a variable restriction 2 is introduced into a nozzle 4. The fluid is then jetted from the nozzle 4 toward a surface of a workpiece 6. A back pressure due to the jet of the fluid, developed behind the nozzle 4 is detected by a sensor S disposed on a line which connects the nozzle 4 and the variable restriction 2.
The back pressure detected by the sensor S and the distance d by which the workpiece 6 is spaced from the head of the nozzle 4 is related to each other as shown in FIG. 2 of the accompanying drawings. Therefore, it is possible to detect whether the workpiece 6 has been positioned on the reference surface by detecting, with the sensor S, the back pressure generated by the jet of the fluid from the nozzle 4 to the workpiece 6.
In the event that the fluid pressure of the pressure fluid source varies, however, the characteristics shown in FIG. 2 also vary, and it becomes impossible to detect accurately whether the workpiece 6 has been positioned on the reference surface.